Hi all,
I am new to the 3D printing world but have designed rapid prototyped parts before. I am trying to have one of my models 3d printed through shapeways but they are saying it can't be done as their are too many shells. I am new to this terminology, and 1. don't fully understand what shells in this context are and 2. don't know how to fix it. Is anyone able to help in this department?

The term shells means the individual watertight meshes that make up your model. As for why the model was rejected for 'too many shells' could be down to a couple of reasons;
1. Checker error. When you order your model for the first time, your model is manually checked by an operator, sometimes they ake mistakes as happened with a chain maille piece I once ordered - the solution to this is to ask for Shapeways to check again with your reasons as to why you think it is a mistake on their part.
2. The shells making up your model do not quite overlap and some shells are floating free of the main model shell. The solution to this is to ensure that you have no free floating shells in your software and that your model is one complete shell, which can be done by using NetFabb Cloud Service

The model can be whatever it needs to be, but the shells making up the model must be watertight - i.e. the planks making up the sides of the truck must have no holes from the outside surface to the inside surface of the shell that makes the plank.